In order to reduce emissions of carbon dioxide resulting from the production process of clinker, the current trend is to reduce the quantity of clinker in a concrete. One possibility consists of at least partially replacing part of the clinker by a cement addition or adding an addition of a type of fly ash to the clinker. An example of a cement addition corresponds to fly ash, which is a waste material produced by coal power stations.
Certain countries impose a reduction of emissions of heavy metals, in particular mercury, in power stations. With this aim, active carbon may be injected in the process to trap the heavy metals and may therefore be found in the waste material produced by the power plants. Active carbon may then be found in the fly ash used as a cement addition. One disadvantage is that active carbon modifies the action of certain admixtures used in concretes, in particular the air-entraining agents.
An air-entraining agent, or AEA, is an admixture which makes it possible to increase the quantity of entrained air in a concrete during the production of the concrete. The air-entraining agent is for example as defined in the NF EN 934-2 Standard, <<Admixture for concrete, mortar and slurry—Part 2: Admixtures for concrete—Definitions, specifications, conformity, marking and labelling>>. The presence of active carbon tends to reduce the action of the air-entraining agent. More generally, the presence of carbonaceous materials in the hydraulic composition, in particular active carbon or unburned carbon in fly ash, tends to reduce the efficiency of the air-entraining agent of a hydraulic composition.
One difficulty comes from the fact that the quantity of active carbon is generally very variable from one type of fly ash to another and may greatly vary for the same type of fly ash. It is therefore not possible to predict the evolution of the efficiency of an air-entraining agent when using a new batch of fly ash for the production of a concrete.
Furthermore, it is possible that a reduction of emissions of heavy metals, in particular mercury, will also be imposed on cement plants that produce clinker. It could then be envisaged that active carbon be used to trap the heavy metals and be found in the clinker produced by the cement plant or in the dusts of a cement kiln. The presence of active carbon would then tend to reduce the efficiency of an air-entraining agent present in the concrete produced using the clinker.
There therefore exists a need for a process of production making it possible to obtain the regularity of the properties of a hydraulic composition, in particular a hydraulic composition having a desired and regular content of entrained air, in the presence of at least one constituent comprising variable quantities of carbonaceous materials, in particular active carbon or unburnt carbon.